PowerPC: Update MIP405/MIP405T to use Kconfig better

Convert CONFIG_MIP405T from SYS_EXTRA_OPTIONS to a real config

There are two boards, MIP405 and MIP405T that have a few differences.
Start by checking for CONFIG_TARGET_MIP405.  Then introduce
CONFIG_TARGET_MIP405T and use that not CONFIG_MIP405T.  Next, convert
also convert the usage of CONFIG_ISO_STRING to be based on Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/powerpc/cpu/ppc4xx/Kconfig b/arch/powerpc/cpu/ppc4xx/Kconfig
index 92a330d..a6066ef 100644
--- a/arch/powerpc/cpu/ppc4xx/Kconfig
+++ b/arch/powerpc/cpu/ppc4xx/Kconfig
@@ -104,6 +104,9 @@
 config TARGET_MIP405
 	bool "Support MIP405"
 
+config TARGET_MIP405T
+	bool "Support MIP405T"
+
 config TARGET_PIP405
 	bool "Support PIP405"